The definition of oxidative stress is an in vivo imbalance between the formation and elimination of reactive oxygen. Changes of the normal redox state in the cell or tissues can produce harmful radicals that may damage components of the cellular machinery, including DNA, proteins and lipids. If the cellular components are chemically altered that cause genetic changes, this has generally been considered to promote formation of cancer or other serious diseases.
Sources of Oxygen Radicals—
Numerous in vivo generators of oxygen radicals (O2−, H2O2 and OH−) that potentially can cause oxidative stress have been identified: complex I and III in the mitochondria and NAD(P)H oxidase, xanthine oxidase, cytochromes P450, metal ions (cobalt, vanadium, chromium, copper and iron) and some organic compounds that can redox cycle.
General Antioxidants—
There also are numerous endogenously cellular antioxidants such as superoxide dismutase (SOD), catalase, glutathione peroxidase, peroxiredoxins and sulfiredoxin. Vitamins provided by the food are also considered as an important part of the protection of the organism from harmful oxygen radicals, and recent discovery of important antioxidants present in many sources of food has increased the arsenal of antioxidants.
Antioxidants as Therapeutics—
It is very clear that some antioxidants can be helpful in preventing diseases and promote health. What is much less clear is what type of antioxidants can be used. Many of the antioxidants present in natural food are redox active. If these types of redox active substances are isolated and provided as complementary pharmaceuticals—this may end up being more harmful than helpful. Clinical trials have shown that untargeted application of antioxidants, which broadly scavenge oxygen radicals, are not only ineffective but may even be harmful. This was illustrated in a study made with sixty-seven randomized trials with 232,550 participants including healthy and patients with various diseases (Bjelakovic G, Nikolova D, Simonetti R G, Gluud C. Cochrane Database Syst Rev. 2008 Jul. 16; (3):CD004183. Epub 2008 Jul. 16).
Thus general antioxidants that are redox active may actually be adding to the cellular damage, by mediating a harmful redox cycle. Other general antioxidants will harmfully block normal cellular in vivo activity necessary to maintain bodily function.
Source and Role of Reactive Oxygen—
What has become increasingly clear is that what is causing excessive production and accumulation of reactive oxygen, in a number of pathological conditions, such as inflammation, type 2 diabetes, diabetes complications, polycystic ovary syndrome, stroke, detrimental neurological conditions and cancer, is not generally leaking oxygen radicals such as complex I or III in the mitochondria—rather it is up-regulated powerful producers of oxygen radicals—that are part of the normal cellular signal transduction system. Thus the definition of oxidative stress need not be oxygen radicals that will irreversibly alter DNA, protein or lipids, but instead increasingly interfere, if up regulated with “normal” signal transduction creating an imbalance on a cellular level that eventually may alter other tissues and whole bodily function. A typical example of this is the metabolic syndrome, connected to vascular disease, diabetes 2, stroke, nephropathy, neuropathy, heart failure and stroke with insulin resistance as the initiating factor (Reaven, “Role of insulin resistance in human disease”, Diabetes 37(12), 1988). Insulin resistance in itself is also part of normal bodily function as a tool to direct storage of energy selectively to a suitable receiving organ. However, when metabolic changes occur, such as in overfeeding, or other disturbances such as acromegaly with excess growth hormone production or malfunctioning leptin as in ob/ob-mice, this will induce a harmful condition with an uncontrolled insulin resistance that may cause organ failure connected to the metabolic syndrome. The common denominator to the uncontrolled insulin resistance is overproduction of local and systemic oxygen radicals (Houstis et al., Nature 440, 2006; Katakam et al., J cereb blood Flow Metab, 2012 Jan. 11).
One of the most interesting candidates for this overproduction is a family of trans-membrane proteins (enzymes), referred to as NAD(P)H oxidase (Nox). There are seven family members of Nox identified (Nox 1-5 and Duox 1-2) that very often are being recognized as a major or key source of reactive oxygen and that also play a major role in a number of cellular events as part of the normal cellular signal transduction system, including proliferation (Brar et al., Am J Physiol Lung Cell Mol Physiol, 282, 2002), growth (Brar et al., Am J Physiol Cell Physiol, 282, 2002), fibrosis (Grewal et al., Am J Physiol, 276, 1999), migration (Sundaresan et al., Science, 270, 1995), apoptosis (Lundqvist-Gustafsson et al., J Leukoc Biol, 65, 1999), differentiation (Steinbeck et al., J Cell Physiol, 176, 1998), cytoskeletal rearrangement (Wu et al., J Virol, 78, 2004) and contraction (Rueckschloss et al., Exp Gerontol, 45, 2010).
NADPH Oxidase and Disease—
Some genetic conditions with decreased NADPH oxidase activity have been identified—defect Nox2 decreases immunologic response to kill and neutralize microbial attacks (Chronic granulomatous disease)—defect Nox3 in inner ear renders defective gravity perception and dual NAD(P)H oxidase Duox2 having deficient enzymatic activity in the thyroid gland gives rise to hypothyroidism.
There is however a much larger list of publications that also seems to grow exponentially, that witness of strong evidence that increased Nox activity is part of or even causative of a number of diseases (Lambeth J D, Review Article “Nox enzymes, ROS, and chronic disease: An example of antagonistic pleiotropy”, Free Radical Biology & Medicine 43, 2007; Takac I et al., “The Nox Family of NADPH Oxidases: Friend or Foe of the Vascular System”, Curr Hypertens Rep. 2011 Nov. 10; Montezano A C, “Novel Nox homologues in the vasculature: focusing on Nox4 and Nox5”, Clin Sci London 2011; Bedard K et al., “The Nox family of ROS-generating NADPH oxidases: physiology and pathophysiology” Physiol Rev. 2007; Camici M et al., “Obesity-related glomerulopathy and podocyte injury: a mini review”, Front Biosci 2012; Nabeebaccus A et al., “NADPH oxidases and cardiac remodeling” Heart Fai Rev. 2011; Kuroda J et al., “NADPH oxidase and cardiac failure” J Cardiovasc Transl Res. 2010; Kuroda J et al., “NADPH oxidase 4 is a major source of oxidative stress in the failing heart” Proc Natl Acad Sci USA 2010; Maejima Y et al., “Regulation of myocardial growth and death by NADPH oxidase” J Mol Cell Cardiol. 2011; Barnes J L et al., “Myofibroblst differentiation during fibrosis: role of NAD(P)H oxidases” Kidney international, 2011; Alison Cave “Selective targeting of NADPH oxidase for cardiovascular protection” Current Opinion in Pharmacology 2009; Albert van der Vliet “Nox enzymes in allergic airway inflammation” Biochimica et Biophysica Acta 1810, 2011; Pendyala S et al., “Redox regulation of Nox proteins” Respiratory Physiology & Neurobiology 174, 2010; Nair D et al., “Intermittent Hypoxia-Induced Cognitive Deficits Are Mediated by NADPH oxidase Activity in a Murine Model of Sleep Apnea” PLoS ONE, vol. 6, Issue 5, May 2011; Chia-Hung Hsieh et al., “NADPH oxidase Subunit 4-Mediated Reactive Oxygen species Contribute to Cycling Hypoxia-Promoted Tumor Progression in Glioblastoma Multiforme” PloS ONE, vol 6, issue 9, September 2011; Sedeek M et al., “Molecular mechanisms of hypertension: role of nox family NADPH oxidase” Current Opinion in Nephrology and Hypertension 2009; Augusto C et al., “Novel Nox homologues in the vasculature: focusing on Nox4 and Nox5” Clinical Science 2011; Briones A M et al., “Differential regulation of Nox1, Nox2 and Nox4 in vascular smooth muscle cells from WKY and SHR” Journal of the American Society of Hypertension 5:3, 2011).
It has been recently shown that the Nox enzymes and particularly Nox 4 and NAD(P)H-oxidase are highly involved in pulmonary fibrosis. The function of oxidative stress in fibrosis are well recognized (Kinnula V L, Fattman C L, Tan R J, Oury T D (2005) Oxidative stress in pulmonary fibrosis: a possible role for redox modulatory therapy. Am J Respir Crit Care Med 172:417-422), as there is a substantial and growing body of evidence indicating that oxidative stress plays an important role in the pathological development of lung fibrosis as well as fibrosis in multiple organ systems (Kuwano K, Nakashima N, Inoshima I, Hagimoto N, Fujita M, Yoshimi M, Maeyama T, Hamada N, Watanabe K, Hara N (2003) Oxidative stress in lung epithelial cells from patients with idiopathic interstitial pneumonias. Eur Respir J 21:232-240). Thus, Nox enzymes and particularly Nox4 appear to be involved also in lung infections, acute lung injury, pulmonary arterial hypertension, obstructive lung disorders, fibrotic lung disease, and lung cancer.
NADPH Oxidase Isoenzymes, Similarities, Differences and Function—
All the seven iso-enzymes of NADPH oxidase (identified) are similar in the way of having NADPH and FAD binding site and six trans-membrane domains and in that they include two heme complexes.
All the NADPH oxidase forms use the same basic mechanism to generate reactive oxygen, but the subcellular localizations and the modes of actions differ significantly.
The reactive oxygen species produced by the enzymatic Nox-family are either superoxide O2− or hydrogen peroxide H2O2.
Nox1 and 2 are constitutively attached to p22phox and to activate the enzyme complex other components such as Rac, p47phox, p67phox are required for full Nox1 activity. Nox2 needs Rac, p40phox, p47phox and p67phox for full activation. Nox1 and 2 generate O2− when activated.
Nox3 also needs to assemble cytosolic proteins to be active (Cheng et al., J Biol Chem, 279(33), 2004).
Nox4 is also associated with p22phox, and is constitutively active in this form. Nox4 activity is, however, regulated through expression—not through assembly or ligand activation, which distinguishes this isoform from other isoforms (Serrander et al., Biochem J. 406, 2007). When induced, Nox4 is generally expressed at higher level than Nox1 and 2 (Ago et al., Circulation, 109, 2004). Nox4 seems to mainly generate H2O2 instead of O2− as the other Nox-variants (Takac et al., J. Biol. Chem. 286, 2011). This makes this isoform unique because H2O2 has the ability to cross membranes and thus to act at longer distance than O2− that has a very short half-life.
Nox5, Doux1 and Doux2 are activated by Ca2+ (De Deken, Wang et al., J. Biol Chem., 275(30), 2000).
Nox4 and Diseases—
The uniqueness of Nox4 in comparison to the other iso forms is also connected to uniqueness as a therapeutic target as it seems to be involved in a number of different diseases when overexpressed.
Nox4 is ubiquitously expressed in many cell-types although at a very low level until induced. It is mainly found in kidney, endothelial cells, adventitial fibroblasts, placenta, smooth muscle cells, osteoclasts and is the predominant Nox that is expressed in tumors (Chamseddine et al., Am J Physiol Heart Circ Physiol. 285, 2003; Ellmark et al., Cardiovasc Res. 65, 2005; Van Buul et al., Antioxid Redox Signal. 7, 2005; Kawahara et al., BMC Evol Biol. 7, 2007; Krause et al., Jpn J Infect is. 57(5), 2004; Griendling, Antioxid Redox Signal. 8(9), 2006). It was found that Nox4 was overexpressed in the majority of breast cancer cell-lines and primary breast tumors. Overexpression of Nox4 in already transformed breast tumor cells showed increased tumorigenicity, and Nox4 was here identified in the mitochondria. Nox4 was suggested as a target to treat breast cancer (Graham et al., Cancer Biol Ther 10(3), 2010). Nox4 mediates oxidative stress and apoptosis caused by TNF-a in cerebral vascular endothelial cells (Basuroy et al., Am J Physiol Cell Physiol vol. 296, 2009). Its adverse effect following ischemic stroke is well demonstrated in animal models and human tissue. Knockdown experiment, of Nox4, dramatically reduced the area of neuronal damage (Sedwick, PLos Biology, vol. 8 issue 9, 2010; Kleinschnitz et al., vol. 8 issue 9, 2010).
Ischemic stroke accounts for approximately 80% of all cases of stroke and is the second leading cause of death in the world. It has been shown with KO mouse, that Nox4 is an effective therapeutic target in acute stroke. In a recent report (Kleinschnitz et al, vide supra) convincing data were published, showing that significant increase in Nox4 activity is the main cause of neuronal cell death that occurs in ischemic stroke. It was shown that upon ischemia, elevated Nox4 activity was induced in human as well as in mouse brain. A highly specific Nox4 inhibitor has the possibilities to be a safe drug. Total Knock-out of Nox4 demonstrates no obvious phenotypes in mice. The KO however dramatically improves neuronal cell survival, in induced ischemic stroke with more than 70% compared to wild type. Treatment of the stroke patient could also be performed without any risk for other types of strokes such as hemorrhagic stroke, thus the treatment could be started without the need for CAT scan.
Further, it was demonstrated through knockdown and overexpression studies in both microvascular and umbilical vein endothelial cells that increased Nox4 activity plays an important role in proliferation and migration of endothelial cells (Datla et al., Arterioscler Throm Vasc Biol. 27(11), 2007). Initially it was believed that Nox2 was responsible for the angiogenic defects in diabetes but the focus has shifted more towards Nox4 (Zhang et al., PNAS, 107, 2010; Garriodo-Urbani et al., Plos One 2011; Takac et al., Curr Hypertens Rep, 14, 2012).
Nox4 play a key role in epithelial cell death during development of lung fibrosis (Camesecchi et al., Antiox Redox Signal. 1:15(3), 2011).
It was demonstrated that siRNA-mediated knockdown of Nox4 significantly reduces NADPH oxidase activity in purified mitochondria from mesangial cells and kidney cortex. The knockdown blocked glucose-induced mitochondrial superoxide generation. It was suggested that Nox4 acts as a central mediator to oxidative stress that may lead to mitochondrial dysfunction and cell injury in diabetes (Block et al., PNAS vol. 106, no. 34, 2009).
It was demonstrated that Nox4 was systemically up-regulated at diet-induced obesity in rats (Jiang, redox rep, 16(6), 2011).
Nox4 has been strongly connected to the pathology in failing hearts. (Nabeebaccus A et al. “NADPH oxidases and cardiac remodeling” Heart Fai Rev. 2011; Kuroda J et al., “NADPH oxidase and cardiac failure Cardiovasc Transl Res. 2010; Kuroda J et al., “NADPH oxidase 4 is a major source of oxidative stress in the failing heart” Proc Natl Acad Sci USA 2010). A connection between increased mitochondrial Nox4 activity and dysfunction of “the aging heart” has been suggested (Tetsuro Ago et al., AGING, December 2010, vol. 2 No 12).
Extracellular matrix accumulation contributes to the pathology of chronic kidney disease. The growth factor IGF-I activity is a major contributor to this process and Nox4 is a mediator in this process (New et al., Am J Physiol Cell Physiol. 302(1), 2012). The connection between chronic activation of the renin-angiotensin and the progression of kidney damage system is well established with Nox4 and Angiotensin II as collaborators in this process (Chen et al., Mol Cell Biol. 2012).
From the above, it thus appears that the Nox enzymes have several functions in the living body, and that they may also be involved in various disorders. Examples of such diseases and disorders are cardiovascular disorders, respiratory disorders, metabolism disorders, endocrine disorders, skin disorders, bone disorders, neuroinflammatory disorders, kidney diseases, reproduction disorders, diseases affecting the eye and/or the lens and/or conditions affecting the inner ear, inflammatory disorders, liver diseases, pain, cancers, allergic disorders, traumatisms, septic, hemorrhagic and anaphylactic shock, diseases or disorders of the gastrointestinal system, angiogenesis, angiogenesis-dependent conditions. It also appears that especially Nox4 has been found to be involved in such disorders. Consequently, it is considered that compounds capable of inhibiting Nox, and in particular compounds capable of selectively inhibiting Nox4, would be of great interest for use in the treatment of diseases and disorders involving Nox enzymes, and in particular Nox4.